The invention relates to a fiber optic gyroscope.
It is known that the contrast of the interference signal generated by a fiber optic gyroscope is adversely influenced by polarization coupling centers in the light fiber path. In some fiber optic gyroscopes, this interference in coupler/fiber coil arrangements to produce polarization is limited to splices between individual fiber segments, and can be compensated, albeit at high construction cost, by a highly exact alignment of the fiber ends during the splicing process to ensure that they are flush with one another regarding the main polarization axes. However, in newer fiber optic gyroscopes of the type disclosed known for example in U.S. Pat. No. 4,440,498 or in German Patent Document DE-OS 3 912 005, having a 3.times.3 coupler and a monomode fiber coil, and operating on the basis of the inherent phase shift at the quadrature point, such contrast impurity centers are present in the monomode fiber strand itself, and cannot be eliminated by splicing the fiber ends to achieve polarization. For this reason the above-mentioned German Patent Document provides for contrast-independent signal evaluation using three-point light-intensity measurement; but this technique fails when the contrast tends toward zero as a result of an unfavorable arrangement of the coupling centers in the fiber coil.
The goal of the present invention is to provide a fiber optic gyroscope which achieves maximum contrast of the interference signal in the monomode fiber coil in a structurally simple fashion, and independently of interfering coupling effects.
This goal is achieved according to the invention by combining a fiber section that produces rotational polarization, with a nonpolarizing, coil-forming monomode fiber strand. Accordingly, the transfer characteristics of the resultant total system are modified so that the phase shift caused by cross coupling in the fiber coil is effectively eliminated by deliberately opposite polarizing rotation in the inserted fiber segment, thus resulting in an interference signal with maximum contrast.
An important feature of the invention is that the required polarization rotation angle can be set to the correct value with very low adjustment cost.
Another important aspect of the invention is the incorporation of the rotationally polarizing fiber section into the monomode fiber strand, without the polarization rotation angle having to be adjusted in advance in a costly fashion before the fiber ends are joined. This is achieved by splicing the fiber section into the coil forming monomode fiber, with the fiber section being in an unadjusted state relative to the polarization rotation angle. The fiber section is then twisted around its lengthwise axis in the completely spliced state until the minimum output signal is achieved, with at least the twisted end of the fiber section adjoining a nominally nonpolarizing fiber. In this manner it is possible to avoid the situation in which the nonpolarizing fiber undergoes reverse rotation of the light that could render the desired contrast maximization ineffective, upon twisting of the spliced fiber section end. For this purpose, in one embodiment a circularly double-refracting fiber is used as the rotationally polarizing fiber section, or preferably a fiber that is linearly double-refracting fiber before twisting.
In another advantageous embodiment of the invention, a monomode fiber section is provided as the contrast-adjusting fiber section and is twisted to produce circular double refraction until the polarization rotation angle is reached, and is spliced endwise in the twisted state. This has the advantage that the contrast-adjusting fiber section and the fiber coil can be made from a single monomode fiber, and those twisting-angle errors caused by manufacturing which cannot be corrected after the splicing process, have only a relatively minor influence on contrast maximization, since the twisting angle required to produce circular double refraction in a monomode fiber section is many times (in the case of quartz glass fibers approximately six times), higher than the polarization rotation angle achieved. Thus, when circular double refraction is adjusted before the fiber section is spliced, highly desirable reduction of adjustment sensitivity is achieved.
To further facilitate manufacturing, especially as regards the minimum number of additional splices required for insertion of the rotationally-polarizing fiber section, the fiber section can be mounted at the end of the fiber coil facing the coupler; that is, using a splice that would be required in any event to connect it directly to the coupler.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.